Recently, in addition to the use of broadcast satellites, communication satellites are being utilized for the transmission of broadcast information. Such satellites transmit information in the form of electromagnetic radiation, hereinafter referred to as microwave signals. Furthermore, these satellites transmit microwave signals which are polarized in various directions.
The direction of polarization is defined as the direction of the electric field vector of the microwave signal. Microwave signals can be linearly polarized in either the horizontal or vertical direction, or circularly polarized. Circular polarization can be considered the combination of two linearly polarized signals of the same frequency, travelling in the same direction, having the same amplitude, which are 90.degree. out of phase from one another. Circular polarization can be further classified as either right-handed or left-handed polarization. The aforementioned satellites transmit microwave signals having both linear and circular polarization.
Accordingly, it is desirable for a microwave signal receiver to be capable of receiving microwave signals comprising any of the various polarizations. Heretofore, prior art receivers were capable of receiving either linearly polarized microwave signals or circularly polarized microwave signals. However, as explained below, such devices required complicated and expensive circuitry to accomplish this task.
For example, one prior art receiver capable of receiving both vertically and horizontally polarized microwave signals comprises a converter having a first rod antenna positioned in a cylindrical waveguide so as to receive vertically polarized microwaves and a second rod antenna positioned in the cylindrical waveguide so as to receive horizontally polarized microwaves. Each antenna is electrically coupled to an amplifier which amplifies the output of the respective antenna. The outputs of the amplifier are electrically coupled to a frequency converter or mixer which converts the microwave signal to a lower frequency (i.e. intermediate frequency). This converter does not provide means for receiving circularly polarized microwaves.
In an example of a prior art receiver capable of receiving microwave signals having both linear and circular polarization, the receiver includes a converter comprising a dielectric plate for transforming circularly polarized microwave signals into linearly polarized microwave signals, a ferrite polarizer for rotating the direction of the linearly polarized signals (i.e. between vertical and horizontal), and a mode transducer for receiving the linearly polarized signals. These elements are disposed in series in a cylindrical waveguide which is designed to amplify the output of the mode transducer and convert these signals to an intermediate frequency.
In operation, as signals having various polarizations traverse the cylindrical waveguide, the signals are first incident on the dielectric plate. The dielectric plate, which has a length equal to 1/4 the wavelength of the circularly polarized signal to be received, converts the circularly polarized signal to a linearly polarized signal. Signals originally having linear polarization are not affected by the dielectric plate. The signals are thereafter incident on the ferrite polarizer which functions to transform the signals, which are all linearly polarized, to either a horizontal or vertical polarization. The uniformly polarized signals are then received by the mode transducer, for example, a rod antenna positioned so as to receive the uniformly polarized signals. The antenna output is amplified and then converted to an intermediate frequency.
While this prior art receiver is capable of receiving signals having circular and linear polarizations, the construction thereof is both complex and expensive. More specifically, the ferrite polarizer must vary the angle of rotation for each linearly polarized signal so that each signal has as polarization corresponding to the polarization of the mode transducer. As the incoming linearly polarized signals can have any angle of polarization with respect to the mode transducer, the magnetic field generating circuit within the ferrite polarizer necessarily becomes a complex device.
Accordingly, there exists a present need for an electromagnetic radiation converter capable of converting either linearly or circularly polarized microwave signals to an intermediate frequency, which has a simplified design as compared to the prior art converters described above.